Necessity of Removing American Football Uniforms From Humans With Hyperthermia Before Cold-Water Immersion

We recruited a convenience sample of 19 healthy, recreationally active, unacclimated men. One participant discontinued testing because of the difficulty of the exercise protocol. Eighteen participants completed the study (Tables 1 and 2). Volunteers were excluded from participating if they self-reported (1) having an injury that impaired their ability to exercise; (2) having any neurologic, respiratory, or cardiovascular disease; (3) taking any medications that may have affected fluid balance or temperature regulation (eg, diuretics); (4) having a sedentary lifestyle, which was defined as exercising less than 30 minutes, 3 times per week²¹ ; (5) having a history of heat-related illness (eg, heat exhaustion) in the 6 months preceding data collection; (6) being ill at the time of data collection; or (7) having cold allergy. All participants provided written informed consent, and the study was approved by the Central Michigan University Institutional Review Board.

Participants reported for 2 days of testing separated by at least 72 hours. They completed their testing sessions at approximately the same time of day. We instructed them to avoid exercise, stimulants (eg, caffeine), and depressants (eg, alcohol) for 48 hours before testing; maintain a normal diet; consistently drink water the day preceding testing; and fast for 2 hours before testing. Participants were instructed to consume a similar meal the night before each testing session. Compliance was self-reported before each testing session.

Before testing, participants voided their bladders completely, and we assessed urine specific gravity (SUR-Ne refractometer; Atago USA Inc, Bellevue, WA). If urine specific gravity indicated that he was hypohydrated (ie, >1.020),²²  he was rescheduled. Euhydrated participants were weighed nude (Defender 5000; Ohaus Corp, Parsippany, NJ). Skinfold thicknesses were measured for the chest, abdomen, and thigh in triplicate and averaged per the methods of Pollack et al²³  (baseline skinfold caliper 12-1110; Fabricated Enterprises, Inc, White Plains, NY). Skinfolds were summed and used to estimate body density²⁴  and percentage of body fat.²⁵  Body surface area was estimated using the equation of Dubois and Dubois²⁶  (Table 1).

Participants donned a heart-rate monitor (Polar Electro Inc, Lake Success, NY) and inserted a rectal thermistor (YSI 4600 precision thermometer with 401 probe; Advanced Industrial Systems Inc, Prospect, KY) 20 cm past the anal sphincter to ensure appropriate thermistor depth throughout testing. Next, they put on a dry American football uniform (Table 3), entered an environmental chamber, and stood on a treadmill (model 1850; Proform Performance, Logan, UT) for 10 minutes to acclimate to the environment.²  During this time, they were familiarized with the thermal-sensation²⁰  and environmental-symptoms¹⁸  scales.

After the acclimation period, we recorded T_rec, thermal-sensation and environmental-symptoms scores, and environmental-chamber conditions (Kestrel Heat Stress Tracker 4400; Nielsen-Kellerman, Boothwyn, PA). Participants walked for 3 minutes at 3 mi/h (4.8 k/h) and then ran at 90% of their age-predicted maximal heart rates for 2 minutes on a standard treadmill at a 0% incline. They repeated this walking-running protocol until their T_rec reached 39.5°C. We continuously monitored T_rec to determine when it reached 39.5°C.

When T_rec reached 39.5°C, thermal sensation was recorded. Participants stopped exercising, stepped off the treadmill, and performed 1 of 2 actions. On the first day of testing, they removed all football equipment (we helped remove the shoulder pads) and clothing with the exception of their undergarments, shorts, and crew socks. The time to remove their clothing and equipment was measured (1.2 ± 0.2 minutes). On the second day of testing, they removed their shoes and waited next to the cold-water tub for the amount of time it took them to remove their equipment on day 1. We chose this clothing-removal order to ensure a similar lag period before CWI on both days. They entered a 1135.6-L capacity, noncirculating water tub (model 4247; Rubbermaid, Atlanta, GA) that was 160.7 cm long, 175.3 cm wide, and 63.5 cm high. They immersed themselves to the neck for the duration of cooling. All other body parts remained immersed during cooling. We started a standard stopwatch (Acu-Rite, Schaumburg, IL) the moment their feet touched the water.

A separate 401 thermistor (Advanced Industrial Systems Inc) was secured 21 cm from the bottom of the water bath to ensure the initial water-bath temperature remained approximately 10°C. We monitored water-bath temperature frequently while participants exercised and added ice if the water temperature substantially increased during testing. However, we ensured that all the ice had melted before participant entry. Participants were instructed to tell us if they started shivering. The water bath was kept in the environmental chamber to minimize transfer time and to simulate the ambient conditions that an athlete might experience while being cooled at an outdoor athletic event in the heat. The water bath was stirred each minute by tracing the skin of the athletes with a metal rod per NATA recommendations for CWI.¹⁵  When T_rec was reduced to 38.75°C (half finished with cooling), participants rated their thermal sensations a third time. They remained in the water bath until T_rec was 38°C. We continuously monitored and recorded T_rec each minute. The exact time to reduce T_rec to 38°C was noted.

When T_rec reached 38°C, participants exited the water bath and rated their thermal sensation and environmental symptoms. They exited the environmental chamber, removed the football equipment (day 2) and rectal thermistor, towel dried, were weighed nude, and were excused. No fluids were given to them at any time during testing. Participants wore the same shorts, undershirt, crew socks, and shoes on both days of testing.

Data were assessed for skewness, kurtosis, and omnibus normality to ensure normal distribution. Separate dependent t tests were used to determine whether differences existed between equipment conditions for urine specific gravity, environmental-chamber temperature and humidity, exercise duration, level of hypohydration postexercise, and T_rec cooling rates. We evaluated CWI duration using the nonparametric Wilcoxon signed-rank test because of the violation of statistical normality.

Separate repeated-measures analyses of variance were calculated to determine whether differences in thermal-sensation scores or water-bath temperatures existed between PADS and NO_pads over time. We summed the scores from the 16-item Environmental Symptoms Questionnaire, thereby creating a new cumulative score,¹⁸  and analyzed the data with a repeated-measures analysis of variance. Sphericity was assessed using the Mauchly test. We made Geisser-Greenhouse adjustments to P values and degrees of freedom when sphericity was violated. Normality was assessed using Shapiro-Wilk tests. If we found interactions or main effects, we used Tukey-Kramer post hoc tests to identify differences between uniform conditions at each time point. The α level was set at .05. We used Number Cruncher Statistical Software (version 2007; Kaysville, UT) for all analyses.

All data are reported as mean ± standard deviation with the exception of CWI duration, which is reported as median and interquartile range. Participants were well hydrated pretesting (urine specific gravity: NO_pads = 1.006 ± 0.006, PADS = 1.003 ± 0.004; t₁₇ = 2.2, P = .04). Environmental-chamber temperature (NO_pads = 40°C ± 1°C, PADS = 40°C ± 1°C; t₁₇ = 0.7, P = .51) and relative humidity (NO_pads = 40% ± 3%, PADS = 39% ± 3%; t₁₇ = 1.9, P = .10), pre-CWI water-bath temperatures (NO_pads = 9.97°C ± 0.03°C, PADS = 9.97°C ± 0.06°C; F_1,17 = 2.3, P = .15), hypohydration levels (NO_pads = 1.5% ± 0.3%, PADS = 1.6% ± 0.4%; t₁₇ = 1.3, P = .22), and exercise durations (NO_pads = 40.8 ± 4.9 minutes, PADS = 43.2 ± 4.1 minutes; t₁₇ = 2.0, P = .10; Figure 1) were similar on each testing day.

The duration of CWI was shorter in NO_pads than in PADS (z = 2.3, P = .01; Figure 1). Therefore, T_rec cooling rates also differed (NO_pads = 0.28°C/min ± 0.14°C/min, PADS = 0.21°C/min ± 0.11°C/min; t₁₇ = 2.2, P = .02). Thirteen of 18 participants (72%) cooled more quickly in NO_pads. During CWI, 12 participants shivered in NO_pads (shivering onset = 4.8 ± 1.2 minutes), whereas 11 participants shivered when PADS were worn (shivering onset = 5.7 ± 1.3 minutes).

We did not observe an interaction between uniform condition and time for thermal-sensation scores (F_2,30 = 0.9, P = .42) or a difference between uniform conditions (F_1,17 = 0.2, P = .67). However, thermal sensation changed over time (F_2,30 = 338.0, P < .001; Figure 2). Thermal sensation pre-exercise was lower than postexercise (P < .05). Pre-exercise and postexercise thermal sensations were higher than the scores reported when T_rec was 38.75°C and post-CWI (P < .05). Finally, thermal-sensation scores were lower when T_rec was 38.75°C than post-CWI (P < .05).

We did not observe an interaction between uniform condition and time for environmental-symptoms scores (F_1,17 = 2.2, P = .15; Table 4). However, we noted main effects of uniform condition and time. Participants experienced less intense environmental symptoms in PADS than in NO_pads (F_1,17 = 5.9, P = .03) and reported less intense environmental symptoms pre-exercise than immediately post-CWI (F_1,17 = 26.6, P < .001).

Experts,¹³  the ACSM,¹⁴  and the NATA¹⁵  have recommended removing PADS from AFPs with EHS before CWI. However, the strength of evidence for these recommendations was low¹³  and based on studies in which participants were immersed while wearing little clothing^27–29  or exercised in hot conditions while wearing football equipment.^2,4,18  To our knowledge, we are the first to provide direct evidence to support the recommendation to remove AFP equipment before CWI.^13–15 

The NO_pads T_rec cooling rates and immersion durations were statistically different from those for PADS because PADS insulate the body.⁵  However, these differences are unlikely to be clinically meaningful for 2 reasons. First, cooling rates in both conditions were more than acceptable (ie, >0.16°C/min).⁹  Second, conduction is the predominant method of heat loss during CWI, and PADS do not prevent the body's access to cold water. For example, an individual wearing PADS during CWI with a T_rec of 42°C would require 4 minutes of additional cooling to reach the recommended T_rec CWI removal temperature of 38.6°C.³⁰  Thus, our current data confirmed the observation of Miller et al¹⁷  that T_rec cooling rates are still considered acceptable⁹  when PADS are worn during CWI.

Whereas football uniforms may be removed quickly in EHS scenarios,¹⁶  the time needed to remove PADS is based on 5 assumptions: (1) trained individuals are present to remove PADS from AFPs with EHS, (2) the individuals treating the AFPs with EHS are familiar with PADS removal, (3) the PADS are removed easily, (4) the necessary tools for uniform removal are available, and (5) AFPs are compliant with medical personnel. The clinical value of our study is in providing useful guidance when any, or all, of these assumptions are not met.

Medical professionals trained to recognize, diagnose, and treat EHS (eg, athletic trainers) may not always be present to treat AFP with EHS. Currently, fewer than 50% of US secondary schools employ full-time athletic trainers.³¹  Thus, coaches, administrators, or other laypeople may be responsible for making medical decisions and initiating treatment.³²  Not having to remove football uniforms before immersing athletes with EHS would simplify the treatment process while ensuring excellent cooling rates.

Sometimes removing football uniforms is difficult (eg, unfamiliarity with equipment, mechanical failures, or unavailability of scissors). Some football equipment can be removed in approximately 3 minutes¹⁶  but only when the removal tools are immediately available and the individuals are medical experts trained in football-uniform removal. Experts³²  have noted that many AFPs with EHS are allowed to rest next to the field of play because lay responders do not recognize EHS and initiate treatment. Failure to remove the football uniform for any of the reasons given is not a concern if lay responders initiate CWI.

The size, temperament, and level of consciousness of an athlete with EHS may also delay football-uniform removal. Most AFPs with exertional heat illness are overweight or obese (eg, linemen, linebackers).⁶  Moreover, athletes with EHS often have central nervous system dysfunction that may manifest as aggression, irritability, or unconsciousness.¹⁵  Therefore, initiating CWI may be easier and safer for individuals treating AFPs with EHS if the athletes are immersed while fully equipped.

Initiating CWI as quickly as possible is imperative, and the best prognosis occurs if CWI is initiated within 30 minutes of symptom development.^7,8  If evaluation or CWI has been delayed longer than 30 minutes, the medical professional may need to initiate treatment immediately while the AFP is fully equipped rather than spend additional minutes removing equipment. Given that the difference in CWI durations was small (approximately 1 minute), it is unlikely that wearing PADS during CWI would result in increased mortality or morbidity due to prolonged hyperthermia.

An ancillary goal of our study was to clarify the observation of Miller et al¹⁷  of faster cooling rates when participants wore PADS during CWI versus minimal clothing. They speculated that the faster cooling of fully equipped participants was due to 1 or more of the following: (1) exercising while wearing different amounts of clothing and equipment, (2) shivering during CWI in the minimal-clothing condition, or (3) greater conductive cooling in the fully equipped condition due to PADS touching the skin.¹⁷  Because participants cooled slower in PADS in our study despite feeling similarly hot, the observation of Miller et al¹⁷  was likely due to skin temperatures being higher after exercise in the fully equipped condition.^2,4,18  Shivering could not explain their results, as the onset of shivering occurred 1 minute sooner in NO_pads. Therefore, shivering may have played only a small role in total CWI duration as a result of the relatively short CWI times.²⁹  Finally, it appears that wearing PADS during CWI did not convey an additional thermoregulatory advantage by providing greater conductive cooling, as illustrated by the longer CWI durations in PADS in our study.

Despite numerous lives being saved by prompt CWI,¹²  researchers have criticized CWI for causing a cold-shock response; harmful, albeit transient, biochemical and physiologic changes (eg, free-radical formation, tachycardia)³³ ; and discomfort to conscious patients.¹⁹  To address whether wearing uniforms during CWI affected comfort, we asked participants about their perceptions of thermal sensation and the presence of environmental symptoms. Their thermal sensations during CWI did not differ between uniform conditions, indicating that PADS did not interfere with their perception of cold. However, PADS decreased some of the discomfort associated with CWI, as indicated by lower environmental-symptoms scores. This decreased discomfort may be due to an insulating effect of the additional clothing.⁵  Whereas patient comfort should never trump optimal cooling methods,⁷  our data suggested that wearing PADS decreased some CWI discomfort while still ensuring acceptable T_rec cooling.

If PADS are worn during CWI, medical professionals must evaluate and ensure the integrity of the cardiovascular system during EHS treatment. If PADS are left on AFPs during CWI and a life-threatening event (eg, cardiac arrest) occurs, emergency providers must remove the equipment before enacting life-saving measures. This concern could be alleviated by starting to remove equipment after the patient has been immersed.

We acknowledge the possible limitations and assumptions of our study. First, we assumed skin temperatures were similar before CWI based on the similar thermal-sensation scores reported postexercise. Thermal sensation is influenced more heavily by skin temperature than body core temperature,²⁰  but skin wetness plays a role in thermal sensation and should be considered when comparing our CWI thermal-sensation data with scores before and during exercise. However, several variables thought to influence the heat storage and T_rec of participants were similar before CWI each day: hypohydration level,³⁴  intensity and duration of exercise,³⁵  and environmental temperature and relative humidity.³⁶  Therefore, we do not believe the lack of skin-temperature data altered our conclusions or interpretation of the data. Second, shivering onset was self-reported and not quantified using oxygen consumption or heart rate.²⁹  Third, a potential order effect may have occurred in our experiment because all participants removed PADS before CWI on the first day of testing. However, the likelihood is small given that the physiologic adaptations to heat (eg, increased skin blood flow)¹⁵  or cold (eg, shivering threshold, thermal comfort)³⁷  require more than 1 day of exposure. Finally, our results may not be applicable to all EHS situations. Researchers should study T_rec cooling rates in fully equipped female athletes and obese or overweight AFPs because sex³⁸  and body size can affect cooling.³⁹ 

Removing AFP uniforms before CWI resulted in higher T_rec cooling rates and should be performed if the following conditions are met: individuals knowledgeable in equipment removal are present, removal tools (eg, scissors) are immediately available, PADS can be removed easily, and PADS interfere with the ability to fully immerse the athlete. If these considerations are not met or CWI treatment is not initiated within 30 minutes of collapse, wearing PADS during CWI with frequent stirring results in acceptable cooling rates. Finally, wearing PADS during CWI did not alter how cold participants felt during cooling but did decrease some of the negative symptoms associated with CWI and hyperthermia.

We thank Michael McPike, MS; Greg McGillvary, MA, ATC; and Joseph Fox, MA, ATC, from the Athletics Department, Central Michigan University, for donating the equipment for this study.